Microporous membranes are well known in the particle filtration industry. The material of these membranes comprises suitable organic plastics such as nylons, acrylics, polyvinylidene fluoride, polysulfone, polyethersulfone, and the like. Their filtration mechanism is a combination of size exclusion (sieving) and absorption or adsorption on the walls of the pores inside the membrane. To be considered "microporous", the typical inner width of the pores is in the range between about 0.03 microns and about 10 microns. Below this range, are "ultrafiltration" (UF) membranes which serve to filter macromolecules rather than particles, or "reverse osmosis" (RO) membranes which serve to separate ions. The smaller the pore size, the slower the rate at which fluids can be passed. To be useful for a particular application, the flow rate of the membrane must therefore be reasonably high.
Microporous membranes may be supported with fibers to provide greater mechanical strength; these fibers may be either woven or nonwoven. Another type of microporous membrane consists of filter paper or depth filters; these are nonwoven webs of cellulose, plastic, glass, metal or other suitable fibers. For the purposes of this invention, the words "microporous membrane" are to be construed in the widest sense.
Cation exchange resins are part of a well known technology. The resins consist of small beads (e.g., 37 to 840 microns in diameter) of crosslinked polymer containing anionic functional groups such as sulfonic acids. The resins can be prepared by suspension polymerization of styrene and divinyl benzene followed by sulfonation. The finished resin beads in use are packed in a suitable column. They are used for many purposes. When water which contains dissolved salts is passed through the column, the cations are exchanged for hydrogen ions (on the sulfonic acid); thus the cations become absorbed on the resin. Carboxylic acids, iminodicarboxylates (which chelate cations), phosphonic acids, sulphates (which hydrolyze) are also useful for ion exchange but are less effective than sulfonic acids.
The uses of membranes vary widely but can be illustrated by one example where such use uniquely solves a practical problem in monitoring salts dissolved in water, e.g., solutes in distilled water used as coolants in nuclear power plants. Metal cations in the cooling water can cause pit corrosion on fuel rods or be transmuted to radioactive atoms; therefore the purity of this water must be monitored carefully.
Since distilled or deionized water has concentrations of cations too low to monitor directly, they are first concentrated by adsorption onto a cation exchanging membrane. Typically several hundred liters of feed water are filtered through a suitable membrane (or series of membranes). The water is often hot (140.degree. F.) and the time for filtration long (typically three days).
The membranes are analyzed for their absorbed cations by some variation of the following procedure. The membranes are digested in hot concentrated sulfuric acid, then concentrated nitric acid, and diluted with deionized water to a clear solution which is then analyzed by atomic absorption or atomic emission spectroscopy. These spectroscopies require aspiration of the solution through a thin nozzle so for aspiration the solution should be free from undigested particles.
Considering this application, a suitable membrane should have the following qualities:
1. High content of strong acid functionality, preferably sulfonic. About 2 microequivalents per 47-mm diameter disc (10.7 square cm. effective surface area) is sufficient, more is preferable.
2. High water flow rate under the conditions of use (high pressure differential); this corresponds to roughly 100 seconds per 100 ml per one atmosphere pressure difference for a 47-mm diameter disc. Faster flow rates (less time to filter 100 ml) would be preferable.
3. Rapid ion exchange. As the water passes through the membrane, its dissolved cations should contact the active adsorbing sites rather than rush past them; this implies that the active sulfonic acid sites should be accessible, located on the membrane surface rather than deep in its interior.
4. Stability in hot water. Neither the base membrane nor its charge modifying materials should hydrolyze, dissolve, distort or deteriorate, nor should they contribute any extractable materials to the filtrate.
5. Low background levels of metal atoms in the membrane. These background metal atoms would interfere with subsequent chemical analysis.
6. Digestibility. The membrane components should dissolve during sulfuric/nitric acid digestion; polytetrafluoroethylene (Teflon), polypropylene, polyethylene, and certain other plastics do not dissolve.
7. Strength. The membrane should have sufficient mechanical strength and durability to withstand stresses from high pressure differences or rough handling.
Quite aside from ion exchange, strong acid membranes have other utilities, which utilize their shape (in the form of sheets or films) and the chemical reactivity of the acid group. One use is for the fabrication of a protective garment, e.g., a protective garment containing a strong acid membrane can be used to decompose and render harmless any of certain nerve gases in its passage through the garment. Another application involves treating the membrane with a solution of a silver salt (silver nitrate) wherein the silver atoms are absorbed (by ion exchange) in the membrane; hydrogen cyanide (gas or solution) passing through the membrane reacts with silver atoms to precipitate as insoluble, relatively harmless, silver cyanide solid. These examples illustrate how the chemical reactivity of the acid group, or of a metal atom bound to the acid group, can be utilized.
In more general terms, a strong acid membrane can be used for any of the various applications for which strong acid resin beads are now used. Membranes are often more convenient because the beads require packing inside a column whereas the membrane is self-supporting.
Strongly acidic, cation exchanging membranes are commercially available. For example, membranes called "Bio-Rex" (Bio-Rad Bulletin 1428, Bio-Rad Laboratories, Hercules, CA) are extremely flexible, strong ion exchange membranes composed of resin beads permanently enmeshed in a polytetrafluoroethylene (PTFE) membrane. A strong acid, cation exchanging membrane (Bio-Rex AG 50) contains 90% by weight of styrene/divinyl benzene, sulfonic acid, resin beads. The nominal ion exchange capacity of AG 50 is 2.63 meq per 47-mm disc.
The PTFE (Teflon) component of such membranes does not digest in sulfuric/nitric acids and so the utility of the membrane is limited. Teflon is not wettable by water so the membranes require an additional wetting agent, like Triton X-100 (or prewetting with methanol or other alcohol); these wetting agents then would need to be extracted before use or the membrane would contribute undesirable extractables to the filtrate.
Another membrane is a weak acid, chelating membrane (Chelex). The active sites are iminodiacetate sites which chelate divalent and trivalent metal cations (but not monovalent cations such as sodium, potassium, ammonium ions) and so the membrane has application more limited than the strong acid membranes.
Also available are ion exchange fibrous sheets (IONEX, Toray Industries, Inc. (Sonoyama, Otsu 520, Japan). These sheets are made of nonwoven filter paper and are not membranes. The Ionex fibers are composite fibers of polyethylene or polypropylene reinforcement dispersed as "islands in a sea of polystyrene." Fibers of polystyrene alone would be too brittle. The polystyrene phase is then chemically modified: sulfonation produces a strong acid fiber. The fibers are fabricated into a nonwoven web or paper. During sulfuric/nitric acid digestion, the polyolefin phase is not dissolved and can clog an aspirator in an AE or AA spectrophotometer. The fiber diameter is typically 40 microns which is small compared to beads but very large compared to a submicron membrane; although the total ion exchange capacity of the fibers is large, the rate of exchange may not be.
U.S. Pat. Nos. 4,012,303, 4,230,549, 4,339,473 (to Vincent F. D'Agostino and Joseph Y. Lee, assigned to the RAI Research Corp.) concern treating solid plastic films, not microporous membranes, by gamma irradiation of solutions of monomers (not preformed polymers) to effect grafting. These films, which are slightly conductive to electricity, are useful as separators in batteries or electrochemical cells.
European Patent Spec. 0 087 228 (assigned to Pall Corp., by Thomas C. Gsell, I.B. Joffee and J.P. Degan) describes how nylon microporous membranes can be prepared by the phase inversion method. This is not a post-treatment of a preformed membrane; the method involves the cocasting of a solution of nylon with another suitable polymer which provides the special surface properties, such as anionic charge. One example is nylon cocasted with polystyrene sulfonic acid (Versa TL, National Starch Co.). Other examples use polymers containing carboxylic acids, and polyethylene imine (PEI, a cationic polymer).
A patent concerning microporous membranes is U.S. Pat. No. 4,604,208 assigned to CUNO by Chaokang Chu, J.V. Fiore, R.A. Knight, P. Marinaccio and A. Roy, issued Aug. 5, 1986, entitled "LIQUID FILTRATION USING ANIONIC MICROPOROUS MEMBRANES." Patents concerning anionic RO and UF membranes are reviewed. The patent describes a process for anionically charge modifying a microporous organic polymer membrane by contacting it with a solution of a charge modifying agent. This agent is a polymer containing acidic functional groups such as sulfonic or carboxylic acids.
U.S. Pat. No. 4,596,858 to Harry P. Gregor, E. Samuelson, P.I. Dalven and C.D. Gregor describes cast films of poly-AMPS crosslinked with glycerol or other alcohols. Typical cure conditions are 140.degree. C. for three hours. The cured, cast films are useful as RO or UF membranes.
U.S. Pat. No. 3,919,154 to Yun-Feng Chang, Mo-Fung Cheung, and Santokh S. Labana describes a paint whose resin is an acrylic interpolymer in which 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) monomer is incorporated as a catalyst in a concentration of less than 1 percent. Hydroxypropyl methacrylate (at 15%) provides the hydroxyl functionality for subsequent crosslinking reaction with a separate melamine-formaldehyde (MF, aminoplast) resin.